1. Field of the Invention
The present invention relates to slope control exposure control apparatus for use in document reproducing apparatus such as planetary microfilmers and, more particularly, where a microprocessor is used to control the shutter time based upon a signal level from a sensor that determines the reflectiveness of a document and a look-up table that controls the shutter operation.
2. Description of the Prior Art
When using a planetary microfilmer, it is desirable to have the capability of automatically exposing the documents to their optimum film density level. Some planetary microfilmers typically use sensors to determine the reflectance of a document. Once the reflectance is known, the shutter speed is controlled to achieve some target density. This may be the same film density for the full range of document reflectances. This does not optimize performance of a printer that produces prints of low reflectance, low contrast documents. It is best to expose the film at a lower film density in the case of a low contrast document. This change in film density as a function of document density is called an exposure slope.
U.S. Pat. No. 3,795,445, issued Mar. 1, 1974, entitled "Slope Control Exposure Apparatus" describes an apparatus for creating a film exposure slope. A portion of the light that is used to illuminate the documents is combined with the light reflected from the document and collected on a photosensor. In this way, the film density will drop as a function of lower document reflectances. This design requires additional optics and involves critical alignment for proper operation.
U.S. Pat. No. 5,146,267, issued Sep. 8, 1992, entitled "Reflectance Based Microfilming Process" describes a process for optimizing film exposure by controlling the illumination level of the document based upon a light meter reading for reflectance. Sample targets of background hues are used to establish calibration points and the operator adjusts the document illumination in accordance with a reference spreadsheet. In the present invention, no sample targets are required so there is no need to create the sample calibration film.